Making a shadeless card
Emission Shader + Alpha masking

Shadeless material in cycles can be obtained by using the Emission shader. For further instruction on how to handle all the interactions with other objects in the scene and dealing with shadow casting, I'll suggest to read: 
Read Node for setting render colour, regardless of lighting

The alpha of the image can be used to control the shading property of the object by using as an input of a Mix Shader that mix the Emission shader and a trasparent shader.

Here's a possible node setup.